Method for sintering ceramic formed bodies

ABSTRACT

A method of sintering ceramic formed bodies, in which a container for sintering is disposed in a sintering space of a sintering furnace, and atmospheric gas is introduced from an atmospheric gas inlet port provided on a bottom surface of the container for sintering into a large number of ceramic formed bodies piled up in the container for sintering, thereby to sinter the large number of ceramic formed bodies piled upon each other, while introducing the atmospheric gas into spaces between the ceramic formed bodies.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method of and an apparatusfor sintering ceramic formed bodies, and more particularly, to asintering method and a sintering apparatus which allow atmospheric gasto be controlled reliably and efficiently.

2. Description of the Prior Art

In mass-provided ceramic electronic components and the like, a largenumber of ceramic formed bodies are usually sintered at the same time.For sintering such a large number of ceramic formed bodies, a tunnelfurnace and a batch type sintering furnace are generally used. Thetunnel furnace is used for sintering ceramic formed bodies which can besintered according to a relatively simple temperature profile andrequires little atmospheric control. On the other hand, the batch typesintering furnace is used for sintering ceramic formed bodies whichrequire special atmospheric control.

Even when ceramic formed bodies are sintered using either one of theabove described sintering furnaces, a large number of ceramic formedbodies are usually piled on a bottom board 1 as shown in FIG. 2A or in acontainer for sintering 3 as shown in FIG. 2B, and the bottom board 1 orthe container for sintering 3 is disposed in the sintering furnace. Inaddition, a plurality of bottom boards 1 are usually piled upon eachother and a large number of ceramic formed bodies (a lump of a largenumber of ceramic formed bodies is indicated by oblique hatching anddesignated by reference numeral 2) are disposed on each of the bottomboards 1, as shown in FIG. 2A, so as to enhance mass productivity.Similarly, a plurality of containers for sintering 3 are piled upon eachother and are put into the sintering furnace, as shown in FIG. 2B,thereby to enhance mass productivity.

Meanwhile, although the bottom boards 1 or the containers for sintering3 are disposed in the above described sintering furnace when specialatmospheric control at the time of sintering is required, atmosphericgas must be uniformly supplied to spaces between a large number ofceramic formed bodies. In addition, gas exchange on the surface of eachof the ceramic formed bodies must smoothly progress. Accordingly, thegas must be uniformly discharged from the spaces between the ceramicformed bodies. In the method in which the plurality of bottom boards 1are piled upon each other as shown in FIG. 2A, therefore, the bottomboards 1 are separated from each other by supports 4, thereby tosmoothly supply the atmospheric gas to the ceramic formed bodies. On theother hand, in the structure in which the containers for sintering 3 arepiled as shown in FIG. 2B, notches 3a for smoothly introducing theatmospheric gas are formed in the upper part of each of the containersfor sintering 3.

In the above described sintering method using the bottom boards 1 or thecontainers for sintering 3, the atmospheric gas is smoothly introducedinto spaces formed between the bottom boards 1 and the containers forsintering 3. However, it is difficult to cause the atmospheric gas touniformly spread over the large number of piled ceramic formed bodies.

More specifically, as shown in a cross sectional view of FIG. 3,atmospheric gas introduced into a lump 2 of a large number of ceramicformed bodies is smoothly supplied to ceramic formed bodies located in aportion (a portion indicated by an arrow A) in the vicinity of thesurface of the lump 2 of the ceramic formed bodies, while not easilyspread over ceramic formed bodies located in a portion indicated by anarrow B, that is, ceramic formed bodies located in a lower central partof the lump 2.

Furthermore, it is difficult to uniformly discharge gas after beingsubjected to gas exchange on the surface of each of the ceramic formedbodies in the lump 2. That is, the gas is smoothly discharged fromspaces between the ceramic formed bodies located in the portionindicated by the arrow A in the vicinity of the surface of the lump 2,while the discharging speed of the gas from spaces between the ceramicformed bodies located in the portion indicated by the arrow B issignificantly lower.

As described above, in the conventional sintering method using thebottom boards 1 or the containers for sintering 3, there arises adifference in atmosphere given between the ceramic formed bodies locatedin the vicinity of the surface of the lump 2 of the piled ceramic formedbodies and the ceramic formed bodies located in the inner part thereof.As a result, a sintered body which is not sufficiently sintered, asintered body in which holes remain and a sintered body whose electricalcharacteristics are degraded are liable to be formed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of and anapparatus for sintering ceramic formed bodies, which allow, in sinteringceramic formed bodies requiring atmospheric control, atmospheric gas tobe uniformly supplied to the surfaces of a large number of ceramicformed bodies and the atmospheric gas supplied to spaces between theceramic formed bodies to be discharged at a uniform speed withoutstaying.

A sintering method of disposing in a sintering furnace a container forsintering containing a large number of ceramic formed bodies to sinterthe ceramic formed bodies according to the present invention ischaracterized in that a container for sintering having an atmosphericgas inlet port provided in its inner wall portion in contact with thelarge number of ceramic formed bodies piled upon each other is used asthe above-described container for sintering, and the ceramic formedbodies are sintered while introducing the atmospheric gas from theabove-described atmospheric gas inlet port.

Furthermore, a sintering apparatus for sintering ceramic formed bodiesaccording to the present invention comprises a sintering furnace havinga sintering space for sintering ceramic formed bodies in its inner partand a container for sintering disposed in the sintering space of theabove-described sintering furnace for containing a large number ofceramic formed bodies, the above-described container for sinteringhaving an atmospheric gas inlet port provided in its inner wall portionin contact with the large number of piled ceramic formed bodies.

In the sintering method and the sintering apparatus according to thepresent invention, the atmospheric gas is supplied from the atmosphericgas inlet port of the container for sintering. Since the atmospheric gasinlet port is provided in the inner wall portion in contact with thelarge number of ceramic formed bodies piled up, the atmospheric gasintroduced is directly supplied to a lump of the large number of ceramicformed bodies, so that spaces between the ceramic formed bodies in thelump become paths for the atmospheric gas. As a result, the atmosphericgas is uniformly supplied to the surface of each of the ceramic formedbodies in the lump of the large number of ceramic formed bodies, and gasexchange on the surfaces of the ceramic formed bodies smoothlyprogresses. Consequently, the gas is discharged from the lump of theceramic formed bodies efficiently.

Accordingly, the large number of ceramic formed bodies can be sinteredreliably and uniformly, thereby making it possible to effectivelyprevent the occurrence of, for example, insufficient sintering anddegradation of the electrical characteristics in a case where they areused as electronic components.

Additionally, the container for sintering is provided with theabove-described atmospheric gas inlet port, and the atmospheric gas issupplied to the lump of the ceramic formed bodies directly from theatmospheric gas inlet port. Accordingly, the atmospheric gas need not beintroduced into the entire sintering furnace. That is, atmosphericcontrol of the entire sintering space of the sintering furnace need notbe performed, thereby making it possible to conserve the amount ofatmospheric gas used. Consequently, it is possible to reduce the cost ofthe ceramic sintered bodies obtained.

Furthermore, the atmospheric gas can be uniformly supplied to the spacesbetween the large number of ceramic formed bodies piled upon each otheras described above, thereby making it possible to fill the container forsintering with more ceramic formed bodies with higher density and sinterthe ceramic formed bodies and to also increase sintering efficiency ofthe ceramic formed bodies.

The present invention is suitably applicable to a method of sinteringceramic formed bodies in general requiring control of a sinteringatmosphere.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view illustrating a sintering apparatus anda sintering method according to an embodiment of the present invention;

FIG. 2A is a perspective view illustrating bottom boards used in theconventional sintering method, and FIG. 2B is a perspective view ofcontainers for sintering used in the conventional method;

FIG. 3 is a cross sectional view illustrating the problems in theconventional method;

FIG. 4A is a vertical sectional view showing a container for sinteringused in the embodiment of the present invention, and FIG. 4B is aperspective view showing the container for sintering as seen from below;and

FIG. 5 is an enlarged perspective view showing ceramic formed bodiespiled upon each other.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 4A and 4B are cross sectional views showing a container forsintering used in a sintering apparatus according to an embodiment ofthe present invention and a perspective view showing the container forsintering as seen from below, respectively. A container for sintering 11has a square cylindrical container body 12 having an opening 11a in itsupper part. An atmospheric gas inlet cylinder 13 having an atmosphericgas inlet port 13a opened toward a bottom surface 12a of the containerbody 12 is provided beneath the container body 12. In addition, aperforated plate 14 having a number of through holes 14a for providing aplurality of gas inlet paths formed therein is disposed in a portion,which faces the bottom surface 12a of the container body 12, of theatmospheric gas inlet port 13a.

Description is now made of a sintering method using the above-describedcontainer for sintering 11 with reference to FIG. 1. In the case ofsintering, a large number of ceramic formed bodies are contained in theabove-described container for sintering 11. In FIG. 1, the large numberof ceramic formed bodies are contained in the container for sintering11, and the upper edge of the piled ceramic formed bodies isschematically indicated by a solid line X. That is, the large number ofceramic formed bodies are contained below the solid line X. As shown inan enlarged manner in FIG. 5, a large number of ceramic formed bodies 16are arranged at random. The container for sintering 11 containing thelarge number of ceramic formed bodies 16 as described above is thendisposed in a sintering space 18 of a batch type sintering furnace 17shown in FIG. 1. The sintering furnace 17 is so constructed that aninlet flow path 17a for introducing atmospheric gas is formed on itslower surface, and the atmospheric gas inlet cylinder 13 of thecontainer for sintering 11 is connected to the inlet flow path 17a,thereby to supply the atmospheric gas in the direction indicated by anarrow Y. Reference numeral 19 indicates heaters, which are provided soas to raise the temperature in the sintering space 18.

In the case of sintering, the temperature in the sintering space 18 israised by the heaters 19 while introducing the atmospheric gas in thedirection indicated by the arrow Y. As a result, the atmospheric gas issupplied to a lump of the large number of ceramic formed bodies throughthe through holes 14a of the perforated plate 14. In the lump of theceramic formed bodies, the atmospheric gas is supplied as indicated by alot of arrows Z, in random directions. That is, the atmospheric gas issupplied through spaces in an irregular shape between the large numberof ceramic formed bodies. The atmospheric gas is thus introduced from alower central part of the large number of piled ceramic formed bodies,and is moved upward through the spaces between the ceramic formed bodiesas described above, thereby making it possible to smoothly supply theatmospheric gas to the surfaces of all the ceramic formed bodies piledupon each other. Moreover, the atmospheric gas supplied is subjected togas exchange on the surface of each of the ceramic formed bodies, to besmoothly discharged upward.

Although in the above-described embodiment, the container body 12 andthe atmospheric gas inlet cylinder 13 of the container for sintering 11are respectively formed in a square cylindrical shape and a circularcylindrical shape, the container body 12 may be formed in another shapesuch as a circular cylindrical shape, and the atmospheric gas inletcylinder 13 may be formed in another shape such as a square cylindricalshape.

Furthermore, the perforated plate 14 is so provided as to supply theatmospheric gas supplied in more directions, as well as to prevent thepiled ceramic formed bodies from being dropped into the atmospheric gasinlet cylinder 13. However, the perforated plate 14 may be omitteddepending on the type and the material of the ceramic formed bodies usedand the inner diameter of the atmospheric gas inlet cylinder 13.

Description is now made of a concrete experimental example in which theabove-described embodiment is applied to a method of fabricating aceramic multilayer capacitor having dimensions of 2 mm by 1.25 mm by 0.7mm and having a capacity of 1000 pF. First, a large number of ceramicformed bodies made of a material mainly composed of BaTiO₃ are prepared.The sintering apparatus shown in FIG. 1 is used. Air is introduced asatmospheric gas from the atmospheric gas inlet cylinder 13 at a speed of0.2 liters per minute, and the ceramic formed bodies are sintered at asintering temperature of 130° C. for two hours, to obtain a large numberof multilayer capacitors. The inner diameter of the atmospheric gasinlet cylinder 13 in the sintering apparatus used is 10 mm.

For comparison, bottom boards 1 shown in FIG. 2A are used, to sinter alarge number of ceramic formed bodies prepared in the same manner as theexample. The bottom board used has dimensions of 70 mm long by 150 mmwide by 5 mm thick. In addition, atmospheric gas is introduced byintroducing air into a sintering space of a batch type sintering furnaceat a speed of 2.5 liters per minute.

100 multilayer capacitors are respectively chosen from a large number ofmultilayer capacitors obtained in the above-described sintering methodsin the embodiment and the conventional example, to measure the variationin capacity. The measurements show that the variation in capacity is 4.0CV/% in the multilayer capacitors obtained in the conventional method,while being significantly reduced to 2.5 CV/% in the multilayercapacitors obtained in the embodiment of the present invention.Therefore, it is considered that in the sintering method according tothe embodiment of the present invention, the atmospheric gas isuniformly introduced into spaces between the large number of ceramicformed bodies and is discharged at a uniform speed.

Furthermore, a comparison between the sintering method according to thepresent embodiment using the atmospheric gas inlet cylinder having theabove-described dimensions and the conventional sintering method usingthe bottom boards having the above-described dimensions proves that thenumber of ceramic formed bodies usable for sintering in the methodaccording to the present embodiment is seven times that in theconventional method, so that a significant number of ceramic formedbodies can be sintered at one time in the present embodiment.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A sintering method for sintering a plurality ofceramic formed bodies disposed in a container, said container disposedin a furnace for sintering, comprising the steps of:introducingatmospheric gas through an atmospheric gas inlet port of said container,said atmospheric gas inlet port being provided in an inner wall portionof said container in contact with the plurality of ceramic formed bodiespiled upon each other in said container, said container having a bottomsurface and said atmospheric gas inlet port being disposed so as to openinto the bottom surface of said container; supplying the atmospheric gasdirectly to surfaces of the plurality of piled ceramic formed bodies bypassing the atmospheric gas through spaces between the ceramic formedbodies; and sintering said plurality of ceramic formed bodies.
 2. Thesintering method according to claim 1, wherein a perforated plate havinga plurality of through holes is disposed on the inner wall portion ofthe container for sintering above the atmospheric gas inlet port,wherein in said step of supplying the atmospheric gas passes through theplurality of through holes to pass the atmospheric gas in differentdirections through the plurality of piled ceramic formed bodies.
 3. Thesintering method according to claim 1, wherein said sintering furnace isprovided with an atmospheric gas inlet flow path, and said atmosphericgas inlet port is connected to the atmospheric gas inlet flow path,wherein in the step of introducing the atmospheric gas flows through theatmospheric gas inlet flow path into said atmospheric gas inlet port. 4.The sintering method according to claim 1, wherein in the step ofsupplying, the atmospheric gas is supplied from an atmospheric gas inletcylinder formed integrally with said container for sintering, a portionof said atmospheric gas inlet cylinder which faces the container forsintering forming said atmospheric gas inlet port.